1. Field of the Invention
The present invention relates to a distance measuring equipment for obtaining a distance to an object by irradiating the object with a pulse beam, receiving the pulse beam reflected from the object and thus measuring a time spent from the irradiation and the reception thereof.
2. Related Background Art
This type of known distance measuring equipment is an optical radar system as disclosed in, e.g., Japanese Patent Laid-Open Publication No.2-228579. FIG. 7 is a diagram illustrating a construction of a conventional distance measuring equipment. A light sending device 1 generates a pulse beam P2 by actuating a light emitting element such as a laser diode, etc. . A clock generator 2, which has its output side connected to an input side of the light sending device 1, generates a clock pulse P1 serving as a generation timing for the pulse beam P2. The light sending device 1 inputs the clock pulse P1. A light receiving device 3 receives the pulse beam reflected from the object 7 irradiated with the pulse beam P2. The light receiving device 3 converts the pulse beam into an electric signal P4. A sample pulse generator 4, which has its input side connected to the other output side of the clock generator 2, counts the clock pulses P1 inputted from the clock generator 2. The sample pulse generator 4, at the same time, generates a sample pulse P3. A sample hold circuit 5 is connected to an output side of the light receiving device 3 and to an output side of the sample pulse generator 4. The sample hold circuit 5 performs sampling of the output signals P4 of the light receiving device 3 by use of the sample pulses P3 of the sample pulse generator 4. A processor 6 is connected to output sides of the sample pulse generator 4 and of the sample hold circuit 5. The processor 6 inputs a clock pulse count value from the sample pulse generator 4 and also an output signal from the sample hold circuit 5. The processor 6 thus detects a distance to the object 7.
Next, an operation of the thus constructed conventional equipment will be explained with reference to FIGS. 8 and 9. FIG. 8 is a diagram illustrating operating waveforms within a clock pulse period of the clock generator 2. FIG. 9 is a diagram illustrating operating waveforms at a time interval when this distance measuring equipment measures the distance once. The clock generator 21 generates the clock pulse P1 at a time interval T longer than a time corresponding to the maximum measured distance. This clock pulse P1 is inputted to the light sending device 1. The light sending device 1 generates the pulse beam P2 in synchronism with this clock pulse P1. The light receiving device 3 receives this pulse beam P2 reflected from the object 7. The light receiving device 3 photoelectrically converts the reflected pulse beam into the electric signal and performs a high-frequency amplification thereof. The output signal P4 therefrom is inputted to the sample hold circuit 5. 0n the other hand, the sample pulse generator 4 counts the clock pulses P1 given from the clock generator 2. The sample pulse generator 4 repeats counting, wherein one period is a predetermined clock pulse count value M larger than a value obtained by dividing the maximum measured distance by a distance resolving power. At the same time, the sample pulse generator 4 generates a sample pulse P3 in which the clock pulse P1 is delayed by a time given by multiplying a minute time .DELTA.T corresponding to the distance resolving power by a clock pulse count value n. The sample hold circuit 5 samples a pulse signal of the reflected beam with the above sample pulse P3. The sample hold circuit 5 holds a signal level thereof up to the next sample pulse P3. This held signal P5 is a signal in which a waveform of the high-frequency reflected pulse beam is frequency-converted into a low-frequency signal. The processor 6 compares the low-frequency output signal P5 of the sample hold circuit 5 with a threshold value Vth for detecting the reflected pulse beam and thus detects signals (A and B in FIG. 9) larger than the threshold value. A distance L between the distance measuring equipment and the object is obtained from a clock pulse count value N of the sample pulse generator 4 at this time in accordance with the following formula: EQU L=N.times..DELTA.T.times.C/2 (1)
where C is the velocity of light. Namely, the distance L is 1/2 of a pulse beam travel obtained by multiplying a light emission-through-receipt time given from the clock pulse count value N by the light velocity C.
This type of distance measuring equipment detects the object by irradiating the object with the pulse beam and receiving the pulse beam reflected therefrom. If the light receiving device receives a pulse beam and it's reflected beam from the other light source exclusive of the reflected-from-the-object beam, however, the light receiving device is incapable of determining whether the relevant pulse beam is reflected from the object or comes from the other light source. Especially when the pulse beam from the other light source is incident thereon at a timing coincident with the sample timing of the sample hold circuit, the sample hold circuit samples the pulse beam signal of that light source. Hence, it follows that the processor performs a distance measuring operation with this pulse beam signal and outputs incorrect distance data. The above-mentioned problem relative to the incidence of the pulse beam from the other light source invariably arises when using this type of a plurality of distance measuring equipments. For instance, this type of distance measuring equipment is mounted on a vehicle and utilized for a system which gives an alarm to keep a safety car-to-car distance by measuring a distance to the foregoing car. In this case, if an oncoming car traveling on the opposite lane incorporates the same equipment, the pulse beam of the distance measuring equipment of the oncoming car is certainly incident on the distance measuring equipment of the self-car. The pulse beam from the oncoming car is a direct beam. Therefore, even when the oncoming car is located far away therefrom, an intensity of the beam incident on the light receiving device is much greater than that of the pulse beam reflected from an ordinary foregoing car. At this time, the pulse beam from the distance measuring equipment of the self-car and the pulse beam from the distance measuring equipment of the oncoming car are generated substantially at the same timing, with the result that the pulse beam from the oncoming car is misdetected. Even when there is no foregoing car traveling on the same lane as that of the self-car, it follows that the alarm is mistakenly given. As described above, when receiving the pulse beam from the light source other than the normal pulse beam reflected from the object, the pulse beam is misdetected. This turns out a serious problem relative to a safety and a reliability of a system when this type of equipment is employed as a sensor of the system for giving the alarm and controlling appliances.